A conventional synchronous motor has a stator with at least one alternating current (AC) stator winding and a rotor with at least one direct current (DC) field winding. Such a motor is normally started by an induction motor action whereby a damper winding or solid poles of the rotor correspond to a squirrel-cage of an induction motor, and with the DC field winding being unexcited. The rotor is accelerated with a torque thus produced until it reaches a rotating speed close to a synchronous speed. The DC field winding is excited with DC at a proper instant, and the motor then pulls into step. During the acceleration a relatively high voltage is induced in the DC field winding since it is rotating at a lower speed than the rotating field from the AC stator winding. In order to limit this voltage and to protect the insulation of the DC field winding, thyristors can be introduced to short circuit the DC field winding. This will cause a high current to flow through the DC field winding during the acceleration. This current will negatively influence the starting torque, and a field discharge resistor is commonly used to limit the current.
Synchronous motors of the above kind are thoroughly known in the art, see e.g. EP 1 071 192 B1; U.S. Pat. No. 3,354,368; U.S. Pat. No. 3,959,702; U.S. Pat. No. 4,038,589; and U.S. Pat. No. 4,422,028.
AT334469 discloses a field discharge resistor that is by-passed when the voltage across it exceeds a predetermined threshold value. The field discharge resistor according to AT334469 keeps causing losses during synchronous operation of the motor when the DC field winding is energized with DC.
In the conventional practice, a starting resistor is separately mounted either on the rotor of the machine itself or, in some cases, externally of the motor. In either case, the starting resistors used heretofore are relatively large, heavy and expensive. The power of a typical rotating starting resistor may be 1 MW for a 20 MW motor, and even though the in-duty time is short, e.g. 10-20 seconds, large amount of thermal energy is dissipated. A certain weight of the resistance material is required to avoid overheating of the starting resistor since the cooling time is too short to transfer away the heat from the starting resistor.
Further, the necessary mounting provisions and space requirements complicate the mechanical design, especially when the starting resistor is mounted on the rotor. This is a particular disadvantage in the case of brushless synchronous motors, where the excitation system and starting control are carried entirely on the rotor, and the necessity for also mounting the starting resistor on the rotor involves considerable difficulty and results in rotors of large size and high cost.